Painted Texture Not Rendering
First of all, always Save All Images in Slots tab (Texture Paint mode).

Secondly, Save or Pack as PNG texture in UV Image Editor before closing Blender.



Images used in blender must exist as a file somewhere on the computer (or network). Saving the images will do that. 

By default blender reads the images used from the place where they are saved. To make the .blend file self contained, with the images included within the project file, then pack the images